APIPA Automatic Private Internet Protocol Addressing

• APIPA (Automatic Private Internet Protocol Addressing) is alternatively known as zero configuration networking or address autoconfiguration.
• It is a TCP/IP standard designed by Microsoft as substitute to DHCP (Dynamic Host Configuration Protocol).
• It is used to automatically configure IP-based hosts when a DHCP Server is unavailable.
• APIPA is a useful solution for computer connected to a small local network (LAN) where all devices are connected on a single physical link.
• Computers that have been configured by APIPA have an IP address that is in the 169.254.0.0 range and have a subnet mask of 255.255.0.0.
• IETF's formal standard for APIPA functionality is RFC 3927 (Dynamic Configuration of IPv4 Link-Local Addresses.
• APIPA is useful in cases where there is no DHCP server and and there is either no previous IP address or the IP address's lease has expired.
• A Windows computer will retain the previous IP address as long as the default gateway replies, but if not, will automatically configure itself a new one.
• A discover message is sent every 3 minutes in an attempt to re-establish communications with the DHCP server.